Genetic recombination

Production of offspring with combinations of traits that differ from those found in either parent
A current model of meiotic recombination, initiated by a double-strand break or gap, followed by pairing with an homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right, above. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left, above. Most recombination events appear to be the SDSA type.

Gene conversion - the process during which homologous sequences are made identical also falls under genetic recombination.

During meiosis, synapsis (the pairing of homologous chromosomes) ordinarily precedes genetic recombination.

As indicated in the first figure, above, two types of recombinant product are produced. Indicated on the right side is a “crossover” (CO) type, where the flanking regions of the chromosomes are exchanged, and on the left side, a “non-crossover” (NCO) type where the flanking regions are not exchanged. The CO type of recombination involves the intermediate formation of two “Holliday junctions” indicated in the lower right of the figure by two X shaped structures in each of which there is an exchange of single strands between the two participating chromatids. This pathway is labeled in the figure as the DHJ (double-Holliday junction) pathway.